1. Field of the Invention
The present invention is generally related to the production of poly-beta-hydroxybutyrate (PHB) using Escherichia coli (E. coli) which has been genetically transformed by a vector carrying the genes coding for the PHB biosynthetic pathway and, more particularly, to the more efficient production of PHB in transformed E. coli.
2. Description of the Prior Art
PHB is an energy storage material produced by a variety of bacteria in response to environmental stress and is a homopolymer of D-(-)-3-hydroxybutyrate which has properties comparable to polypropylene. Because PHB is biodegradable, there is considerable interest in using PHB for packaging purposes as opposed to other plastic materials in order to reduce the environmental impact of human garbage. PHB also has utility in antibiotics, drug delivery, medical suture and bone replacement applications. PHB is commercially produced from Alcaligenes eutrophus (A. eutrophus) and sold under the tradename Biopol.
As described in the above incorporated patent application and in the article by Slater et al., "Cloning and Expression in Escherichia coli of the Alcaligenes eutrophus H16 Poly-.beta.-Hydroxybutyrate Biosynthetic Pathway", Journal of Bacteriology, Vol. 170, No. 10, Oct. 1988, p. 4431-4436, which is also herein incorporated by reference, it was shown that E. coli could be genetically transformed with genes from A. eutrophus which code for the PHB biosynthetic pathway. E. coli are a far better vehicle for producing PHB than A. eutrophus since more is known about handling the bacteria, E. coli, i.e., E. coli is more easily controlled and manipulated. The transformed E. coli were able to express PHB in relatively large quantities.
Despite PHB's advantages over other materials, its high cost of production has hindered its performance in the market. Currently, PHB is produced in transformed E. coli by growing the E. coli on luria broth (LB) and using glucose as the carbon source. Approximately one third of the production cost of PHB is attributable to the cost of the rich LB medium and the glucose. If a less expensive carbon source could be utilized, the overall cost of PHB production could be significantly reduced. In addition, much of the total cost of PHB production is attributable to purifying the PHB produced in the E. coli. Currently, PHB is purified by centrifugation, followed by mechanical lysis of the cells to release PHB, a high temperature procedure to agglomerate the PHB, and finally a spray drying step to procure the purified granules. If a less expensive method were available for collecting the PHB from the E. coli the overall cost of PHB production could be significantly reduced.